Manufacture of electric resistance elements



APTil G. c. GAUT ET AL 2,195,531

MANUFACTURE 015' ELECTRIC RESISTANCE ELEMENTS Filed July 22, 1936 GEOFFREY CHARLES GAur I BENJAMIN FREDERICK Husmms BY qrromvsv' "Patented Apr.- 2, 1940 v UNITED STATES PATENT OFFICE MANUFACTURE OF ELEUIRIO RESISTAN ELEMENTS Geoffrey Charles Gaut. Worcester. and Benjamin Fredericklluggins, Forest Gate,lLondon, England Application July 22, mas-serial No. 91.986

Great Britain August 2, 1935 1 acmms- I 'his invention relates to the manufacture of electric resistance elements and variable resistances and has for its object to provide an improved resistance and an improved method of manufacture therefor such that the ohmic value of the resistance and its grading in the case of a variable resistance) can be very accurately controlled.

The primary, feature of the-present inven- 6 tion resides in the use of a conductive film or the amount "of carbon or equivalent conductor incorporated therein and the amount of reduced metal compound.

A further feature of the present invention resides in the manner of graduating the resistance element for use in a variable resistance by control of the size and shape of the area subjected to activation, while a further feature of the invention concerns the provision of a process for producing resistance elements according to the invention in a continuous manner.

In one convenient way of carrying the invention into effect we utilise a suitable base or support which may be Celluloid, cellulose acetate, pa-

3 per or the like and to this we apply a coating I which will adhere to the base. This coating consists of a photo-sensitive compound in which is incorporated a suitable conductor as described ,above. The photo-sensitive material carried by m the base or support is then subjected to the action of actinic light, use being made of a suitable stencil plate or mask if it is desired to graduate the resistance. The exposed element is then subjected to development, fixing, etc. similarly to an ordinary photographic plate whereupon a resistanceelement having values determined by the composition of the photo-sensitive compound and the degree of exposure to light is produced.

Such elementsv are of particular use in connec- 00. tion with the manufacture of variable resistances or potentiometers. 1

In order that the invention may be more readily understood reference will now be made to the accompanying drawing'wherein,

58 Fig.- 1 is a plan view of a suitably apertured ment shown in Fig. 2.

Upon development mask for producing a graded type resistance element;

Fig. 2 is a view of a resistance element produced with the aid of the mask shown inFig. 1;

Fig. 3 represents a schematic layout of ap- 'paratus-for carrying out the method of themvention in a continuous manner;

Fig. 4 is a perspective view of a variable resistance or potentiometer incorporating the ele- A suitable base or support Ill (Fig. 2) consisting of Celluloid, cellulose acetate, paper or the like is provided with a coating of photos-sensitive compound which will adhere to the said base.

'As an example, in the case of a Celluloid base we .II

have found the use of a lacquer consisting of nitro-cellulose containing thephoto-sensitive compound (silver bromide) and the conductor (carbon) to be satisfactory. sensitive lacquer for application to a Celluloid or 5 like base can be made as follows:-

Grams Pyioxylin (high viscosity) "C 1.5 Silver nitrate 11, Ammonium bromide 6.25 a Carbon (light soot") .75 v v Cubic centimeters Ethyl alcohol- 21 Ammoniacal ethyl alcohol 4 Methyl alcohol 4 3o Ether. 12

Acetone '6 Diacetone alcohol 0.5 Triacet 1 In preparing such a lacquer we may prepare a mixture of all of the ingredients except the .ammonium. bromide, silver nitrate and carbon.

The ammonium bromide and silver nitrate are added in powder form in small quantities at a time to the mixture and finally the carbon is 4-0 added, the whole mixture being continuously stirred with stainless steel or glass'stirrers during the addition of these three components. The addition of the first two components under stirring may occupy about one hour and of the third about half an hour. 'Inthis and the subsequent stages of the manufacture due precautlons must obviously be taken to prevent any 'undesired influence due to light. The small solubility of-silver nitrate and ammonium bromide in alcohol permit these two components to react together completely to form silver bromide, a

which, in the presence of the dispersed nitro cellulose, is produced as a colloidal solution. The

' size of the silver bromide particles can be varied A suitable photocontaining pure alcohol.

in known photographic manner by the presence of ammonia and by the time and temperature of the stirring operation. Triacetin is employed as a plasticiser. Small quantities of resin and ester cury discharge tube or are lamp, a suitable stencilled plate or mask such as shown at H in Fig. 1 or an optical wedge being usedwhere necessary to control the light falling on the coated base or carrier 50. The exposed element is then subjected to development, fixing, etc. similarly to an ordinary photographic plate; washing should be effected in pure alcohol. The exposed element is preferably passed through a bath of ammonium bromide in alcohol to prevent fogging before it is developed. The developer used may consist of pyrogallic acid in ammoniacal alcohol while fixing may be effected by a bath of ammonium thiocyanate in ammoniacal alcohol. In this manner the resistance element shown in Fig. 2 is produced, the ohmic resistance of which is determined by the composition of the photo-sensitive lacquer and the degree and extent of activation of the photosensitive compound while the graduation thereof is controlled by the configuration of the mask ll shown in Fig. 1.

While it is possible to manufacture elements by treatment one at a time, e. g. with an optical wedge, we prefer to provide for a continuous process in' order to speed up the output. Referring now to Fig. 3 which illustrates in a schemat'ic manner such a continuous process use is made of a band 13 of Celluloid, paper or the like which is fed from a supply. drum I4 over a jockey pulley E5 to a drum l6 forming part of a constant-level emulsion bath l! which is fed with suitable emulsion, for instance, that previously described, from a supply tank l8. The band I3 is thereby coated with emulsion to the desired extent and is subsequently fed on to the oute'rsurface of an exposure drum l9. In the movement of the band l3 around the axis of this drum I 9 it passes into a lamp chamber where it is subjected to the action of actinic light from a suitable source 2l such as a mercury discharge tube or are lamp so as to activate the photo-sensitive component or components of the emulsion. The band I3 is then passed by way of suitable rollers to a bath 22 containing an anti-fog solution such as ammoniumbromide in alcohol and from this bath the band I3 is passed by way of further guide rollers 23 to a washing tank 24 preferably From this tank the band I3 is directed over guide rollers 25 to a developing tank 26 containing a suitable developer such as pyrogallic acid in ammoniacal alcohol and then over rollers 21 to a further washing tank 28. From this tank it passes over further rollers 29 to a tank 30 where fixing is efiected preferably by means of a bath of ammonium thiocyanate in ammoniacal alcohol from whence itis'directed over rollers 3| through a further washing bath 3m and a drying tunnel or cham-' ber 32 to a roller 33. All of the rollers, as well'asthe drum 1!, may be driven from a suitable drive source (not shown).

The band l3 after passing over the roller 33 is delivered to an automatically operated guillotine 36 where it is cut transversely into sections each constituting a resistance element of specified value determined by the composition of the photo-sensitive lacquer and the duration and extent of exposure to light.

If the resistance desired is to be of the graded type such as a logarithmic or other element for use in a variable resistance, a mask H having supports Ila to the lamp chamber 20 as shown in Fig. 1 is interposed between the light source 2| of the lamp chamber 20 and the strip l3. In this case the exposure varies transversely of the band in accordance with the configuration of th'e mask so that at the upper edge in Fig. l the photo-sensitive lacquer is subject to maximum exposure during the travel of the band while near the lower edge it is subject to minimum exposure, the extreme lower edge however being completely exposed. The resulting conductivity is shown by the graduated shading in Fig. 2, the dotted lines indicating how the band is subsequently cut into strips In by guillotine 33. The lower conductive section forms a low-resistance contact element for contact making purposes during the subse quent mounting of the element, the other contact being of course made to the top of the strip. Either before or after severing into strips the band I3 is preferably polished, e. g. with soft filter paper or like pad as indicated at 34.

Instead of controlling the conductivity by varying the exposure we may control it by suitable choice of wavelength. For example, by using a prism (preferably quartz) in conjunction with a source of substantially white light we may project a spectrum on to the band.

In place of a separate mask we may use a supporting band of varying opacity and illuminate through the band.

While we have referred to the use of a support coated with photo-sensitive lacquer it is clear that the latter need not form a permanent fixture on the support. It may be removed from the support after treatment or, if sufliciently strong, processed without the aid of a support.

Fig. 4 shows a variable resistance or potentiometeremploying such 'a resistance element and comprising a hollow cylindrical b0dy 38 around the inner circumferential surface of which is,

' the pressure arm 40 causes the button 42 to move around the band 39 to press that part of. it lying beneath the button 42 against the resistance element so that any portion of such element between the two ends 43, 44 may be connected to the fiexible band 39. These ends 43, 44 of the strip ID are each connectedto external contact making means such as soldering contacts 45 in known manner.

We claim:

1. The method of making an electric resistance element for use with a movable contact in adjustable resistance devices which includes the steps of forming a sensitized photographic layer having a conductor in finely divided form incorporated therein, the photosensitive ingredient in said photographic layer being of a type which forms a permanently conductive material upon suitable photographic exposure and development,

exposing said layer to actinic radiations adapted to affect, said photosensitive ingredient and controlling the exposure to produce a predetermined gradation of photographic efiect across said layer, and developing and fixing said layer to produce said resistance element having a graduated resistance value across said layer.

2. The method of making an electric resistance element for use with a movable contact in adjustable resistance devices which includes the steps of forming a sensitized photographic layer having a conductor in finely divided form incorporated therein, the photosensitive ingredient in said photographic layer being of a type which forms a permanently conductive material upon suitable photographic exposure and development, exposing said layer to actinic radiations adapted to affect said photosensitive ingredient and interposing a mask of desired graded light transmitting characteristics and shape in the path of said radiations to produce a predetermined gradation of photographic effect across said layer, and developing and fixing said layer to produce said resistance element having a graduated resistance value across said layer. I

3. The method of making an electric resistance element for use with a movable contact in adjustable resistance devices which includes the steps of forming a sensitized photographic layer having finely divided carbon and a photographically sensitive silver compound incorporated therein, exposing said layer to radiations adapted to photographically afiect said silver compound and controlling the exposure to produce a predetermined gradation of photographic efiect across said layer, developing said layer to reduce said silver compound 'to silver in a graded density corresponding to saidexposure and fixing said developed layer.

4. The method of making an electric resistance element which includes the steps of forming a sensitized photographic lacquer comprising nit'ro-cellulose, a photographically sensitive silver compound and finely divided carbon, applying said lacquer as a layer onto a base of cellulosic material, exposing said layer to radiations adapted to photographically afiect said silver compound and controlling the exposure to produce a predetermined gradation of photographic effect across said layer, developing said layer to reduce said silver compound to silver in a graded density corresponding to said exposure and fixing said developed layer. I

5. A continuous process for manufacturing electric resistance elements which includes the steps of feeding a band of sheet material having a layer of photographically sensitive compound thereon under a beam of radiations adapted to photographically afiect said sensitive compound whilecontrolling the amount of radiations to effect a gradation across said layer or the total radiation reaching the various lateral parts thereof, subsequently passing said band through developing, washing, fixing and drying means, and finally cutting said band transversely into a plurality of strips 01 graded linear resistance.

6. The method of making an electric resistance element for use with a movable contact in adjustable resistance devices which includes the steps of dissolving pyroxylin, silver nitrate and ammonium bromide in a common solvent together withan insoluble conductor in finely divided form suspended in said solvent, depositing the mixture in a thin layer on a sheet base, exposing said layer to actinic radiations adapted to affect the silver salt in said layer and limiting the exposure to different extents at different points across said layer to produce a predetermined gradation of photographic effect across said layer, and developing and fixing said layer to produce said resistance element having a graduated resistance value across the said layer.

7. The method of making an electric resistance element for use with a movable contactin adjustable resistance devices which includes the steps .of dissolving pyroxylin, silver nitrate and ammonium bromide in a common solvent together with an insoluble conductor in finely divided form suspended in said solvent, depositing the mixture in a thin layer on a sheet base, exposing .sistance value across said layer.

8. The method of making an electric resistance element for use witha movable contact in adjustable resistance devices which includes the steps of forming a sensitized photographic layer of pyroxylin, silver bromide and a uniformly distributed conductor in finely divided form, ex-

posing said layer to actinic radiations adapted to affect said silver bromide and limiting the exposure to different extents across the said layer to produce a predetermined gradations of photographic e'ffect across said layer, and developing and fixing said layer to produce said'resistance element having a graduated resistance value across said layer. v

9. The method of making an electric resistance element for use 'with a movable contact in adjustable resistance devices which includes the steps of forming a sensitized photographic layer of pyroxylin, silver bromide and a uniformly distributed conductor in finely divided form, exposing said layer to actinic radiations adapted to aflect said silver bromide and limiting the exposure to different extends across said layer to produce a predetermined gradation or photographic eflect across said layer, and developing said layer in pyrogallic acid in ammoniacal alcohol and fixing said layer to produce said resist- GEOFFREY CHARLES GAUT. BENJAMIN FREDERICK HUGGINS. 

